The oldest known method in the art of utility pole construction is the use of wooden poles, such as those commonly used for telephone lines. However, many modern utility pole uses require longer lengths than are practical, or even possible, with wood, and wood poles are highly susceptible to rot, insect infestation, bird attack and high winds. Additionally, construction of wooden poles requires that the pole be of one piece of uncut wood which creates difficulties in transporting and erecting long poles. To overcome the shortcomings of wood utility poles, solid concrete poles are often used. Concrete utility poles, however, are expensive to produce, heavy and require special heavy duty equipment to load, transport, unload and install. Further, the greater weight of concrete poles precludes the use of very long poles. Metal poles have long served as an alternative to wood and concrete utility poles. Metal poles are relatively strong and capable of being constructed in sections for ease of transportation and erection. The widespread use of metal poles however has been limited since steel poles are expensive to produce and aluminum alloy utility poles do not have sufficient strength to be used in high lateral force environments.
More recently, the concept of using ductile iron for utility pole construction has been suggested. It is believed that ductile iron utility poles will provide a virtually maintenance free, extremely long life, low cost utility pole. An exemplary ductile iron utility pole is disclosed in U.S. Patent Application Publication No. 2008/0023172 A2 to Waugh. The pole is a centrifugally cast utility pole having a tapered exterior and a substantially uniform wall thickness along the long axis of the pole. Additionally, U.S. Pat. No. 5,784,851 to Waugh discloses a hollow, centrifugally cast, utility pole having tapered external linear dimensions. The pole is formed utilizing conventional centrifugal casting methods wherein a tapered mold is used to impart a tapered shape to the pole. The use of the tapered mold during the casting operation also provides for a gradually increasing pole wall thickness along the entire length of the pole from the top of the pole to its butt.
Metal utility poles such as the centrifugally cast, tapered poles described above are fabricated to provide for a press-fit, slip joint at the butt of the poles which allows the poles to be interconnected with other similarly cast poles for extended height. To provide such a joint, it is important that bottom ends of the poles have a tapered interior surface that is complimentary with the tapered exterior top ends of the poles to ensure full contact between the top ends and the bottom ends of the poles when they are interconnected. Obtaining the desired taper along the exteriors of the top ends of the poles is a relatively simple endeavor since the exteriors of the poles are determined by the inner surfaces of the molds in which they are formed. Obtaining a desired tapered interior surface along the bottom ends however is more difficult.
The conventional method for providing a centrifugally cast metal pole with a desired bottom end inner wall shape and diameter utilizes a sand core that is inserted into the bottom end of a pole mold during centrifugal casting of the pole. The sand core includes a continuous wall that projects into the mold a predetermined length and at a predetermined distance from the mold wall thereby forming a cavity between the mold wall and the core wall. This cavity ultimately defines the inner shape and diameter of the bottom end of the pole. During the casting, molten metal is introduced at the opening of the cavity, and the mold is rotated to centrifugally force the molten metal down into the cavity. It has been found, however, that this method is often insufficient for controlling the shape and inner diameter of the bottom end of poles since the joint length required for interconnecting utility poles requires the use of a core wall and cavity lengths that mold rotation and centrifugal force alone cannot completely fill with molten metal. For example, the conventional press-fit joint length between hollow metal pole members is about 1.5 times as large as the inner diameter of the butt of the pole. Thus, the core wall of a sand core used to form the interior taper of the bottom end of ductile iron pole must project a length into the mold equal to 1.5 times the inner diameter of the butt of the pole to be formed therein.